1. Field of the Invention
The present invention relates to a method of forming wiring on a substrate, and more particularly to a method of printing the wiring on the substrate.
2. Description of Related Art
Currently there are three different ways of encapsulating a drive IC (integrated circuit) on a liquid crystal display (LCD), namely, 1, a Tape Carrier Package (TCP); 2, a Chip On Flex (COF); and 3, a Chip on Glass (COG). The first method and the second method require a soft circuit board to function as the carrier base board to combine the glass plate and the printed circuit board (PCB) on the display. The soft circuit board for the driver IC is divided into three categories:
The first soft circuit board:
With reference to FIGS. 1A and 1B, it is noted that the very bottom is a copper layer (51). The second layer is an adhesive layer (52) and the third layer is a soft polyimide (PI) layer (53). In particular, the structure shown in FIG. 1B indicates that besides the structure being the sane as that of FIG. 1A, a second adhesive layer (52a) is attached to a free side of the polyamide layer (53), and a second copper layer (51a) is then attached to a free side of the second copper layer (52a). While forming the wiring on the copper layer (51), due to the etching process required to form the wiring on the copper layer (51), the thickness of the copper layer (51) has to at least be 12 μm due to the handling issue so that the effective product yield rate is high. However if the copper layer thickness is big, it will be very difficult for making slender wiring. For example, if the copper layer thickness is big, and the wiring has an average of 40-45 μm pitch (25 μm in width and 20 μm in-between), the effective product yield rate becomes low and that is why the current standard in industrial business is at the range of 45 μm pitch. When the wiring pitch is below 45 μm, a copper film having such as 12 or 9 μm thickness is required. Nevertheless, using a copper foil of this thickness still has its drawbacks. That is, the thin copper foil wrinkles easily during the wiring forming process and this causes a high defect rate. Therefore, the cost is high.
The second soft circuit board:
With reference to FIGS. 2A and 2B, it is noted that there is no adhesive layer when compared with the embodiment in FIGS. 1A and 1B. The structure comprises a polymide substrate (54), a Monel layer (alloy of nickel and copper) sputtered on a surface of the polyimide layer (54) and a copper layer (55) attached to the Moriel layer. Thereafter, the thickness of the copper layer (55) is controlled by plating to reach 6 or 9 μm. With this method, it is not required to press the copper foil on the polyimide (PI) substrate (54) such that the effective product yield rate is high at fine pitch circuitry such as lower than 40 μm pitch. Furthermore, due to the thickness of the copper foil being controlled by plating, the operator is able to make wires with the required pitch and width, which often reaches a standard well below 30 μm. However, because there is no adhesive layer used to bond the metal layer and the soft polymide substrate, the metal layer sometimes peels off the substrate. Furthermore, the plated copper layer breaks easily so that the reliability of the encapsulated element is unsatisfactory. A further disadvantage is that the Monel and Cr layer is not easily etched away due to a good chemical bonding on Ri film such that electricity leakage occurs.
The third soft circuit board:
The third soft polyimide substrate is achieved by applying a polyimide (PI) directly onto the copper foil to create a substrate similar to the substrate without an adhesive layer. This kind of substrate has problems such as the contraction of copper foil thickness in the curing process and being inapplicable to a thickness under 12 μm. As a result, this product has little market share.
To overcome the shortcomings, the present invention tends to provide an improved wire forming method to mitigate the aforementioned problems.